Overview of Clusters

A cluster is a named collection of server instances that
share the same applications, resources, and configuration information. You can group
server instances on different machines into one logical cluster and administer them
as one unit. You can easily control the lifecycle of a multi-machine cluster with
the DAS.

Clusters enable horizontal scalability, load balancing, and failover protection.
By definition, all the instances in a cluster have the same resource and application
configuration. When a server instance or a machine in a cluster fails, the load balancer
detects the failure, redirects traffic from the failed instance to other instances
in the cluster, and recovers the user session state. Since the same applications and
resources are on all instances in the cluster, an instance can failover to any other
instance in the cluster.